WebGL does not require OpenGL implementation.
Most browsers on Windows use DirectX instead and the games consoles also only have their own 3D APIs.
WebGL is an OpenGL implementation. On Windows, it's often implemented with ANGLE, also an OpenGL implementation.
The GL spec doesn't require that the hardware or OS manufacturers are the ones to implement it. :)
So it's arguably not so much "OpenGL is going away" as it is "OpenGL is going to be maintained by the Safari team, and JS will be the only interface to it" :)